A radioactive liquid waste containing mainly a large amount of sodium nitrate (NaNO3) as an inorganic salt is generated from nuclear facilities, particularly reprocessing plants of spent fuel. The progress has been made in the development of a separation and removal technology in which sodium is separated from sodium nitrate in this liquid waste.
For example, in the United States Department of Energy (DOE), a study has been made on the method in which sodium ions (Na+) are separated from a radioactive liquid waste containing sodium nitrate as a main component using a Na ionic conductor membrane (Na super ionic conductor (NASICON) membrane) or a polymer cation-exchange membrane. It is demonstrated that the Na ionic conductor membrane or the organic cation-exchange membrane enables recovery of sodium ions as sodium hydroxide (NaOH).
As the technology in which radioactive nuclides contained in a radioactive liquid waste are separated from sodium nitrate, there has been used a method in which main nuclides are respectively coprecipitated by adding a large number of reagents to a liquid waste and then a solid is separated through a ultrafiltration membrane. Since a lot of reagents are added in this method, the amount of the liquid waste increases. In addition, the limit of a decontamination factor (DF), which indicates the degree of removal of the radioactive nuclide with respect to a specific nuclide, is about 100 and the efficiency of removal of the radioactive nuclide in the radioactive liquid waste was low.
As the technology in which an attempt is made to reduce the volume of a radioactive liquid waste (concentrated low-level liquid waste) containing, as a main component, sodium nitrate generated from the reprocessing process, an electrodialysis method using a sodium ion permselective membrane is exemplified. This technology may enable separation and recovery of only sodium from the radioactive liquid waste, thus resulting in volume reduction of the radioactive liquid waste. Therefore, a treating test of a liquid waste using a polymer membrane (Nafion®, etc.) or an inorganic membrane (NASICON membrane) is performed.
As the technology in which sodium is recovered from a liquid waste containing a radioactive substance and/or a liquid waste containing no radioactive substance by an electrodialysis method using a sodium ion permselective membrane, papers such as Non-Patent Documents 1 to 5 have been published, and are currently known technologies.
Non-Patent Documents 1 to 5 disclose technologies in which sodium is recovered from a radioactive liquid waste (concentrated low-level radioactive liquid waste) containing sodium nitrate, or a liquid waste containing sodium generated from pulp mills by an electrodialysis method using a NASICON membrane.
In these technologies, sodium ions are migrated from an anode chamber to a cathode chamber by feeding a liquid waste to be treated to the anode chamber, filling the cathode chamber with an aqueous sodium hydroxide solution, and performing electrodialysis using a NASICON membrane as a diaphragm between the anode chamber and the cathode chamber of an electrolytic cell.
Non-Patent Documents 1 to 4 report that only sodium can be separated and recovered from a radioactive liquid waste containing many kinds of radioactive nuclides by separating only sodium without containing Cesium 137 (Cs-137) which belongs to the same group as that of sodium.
As the technology in which an attempt is made to reduce the volume of a radioactive liquid waste, the following technologies are disclosed.
For example, there is disclosed a technology in which an attempt is made to recover sodium and an acid from a radioactive liquid waste containing a high concentration of a sodium salt, thereby reducing the volume of this liquid waste and recycling the sodium and acid (see, for example, Patent Document 1). In this technology, electrodialysis is performed in a state where two bipolar membranes are disposed between an anode and a cathode and then an anion-exchange membrane is disposed between these bipolar membranes at the anode side and a sodium ion permselective membrane is disposed between these bipolar membranes at the cathode side, respectively. Thus, it is possible to separate and recover sodium ions from the radioactive liquid waste containing a sodium salt as sodium hydroxide, and to separate and recover anions from the radioactive liquid waste containing a sodium salt as an acid.
There is also disclosed a technology in which an attempt is made to remarkably reduce the volume of sodium nitrate, as a main component of a concentrated low-level radioactive liquid waste, generated from reprocessing plants without generating NOx which applies a burden on a waste gas system, and to close a radioactive waste processing system by recycling a decomposition product (see, for example, Patent Document 2). In this technology, the concentrated low-level radioactive liquid waste generated from reprocessing plants of spent fuel is fed to an electrolysis cell including a cation-exchange membrane and an anion exchange membrane. Thus, it is possible that sodium hydroxide is formed at the cathode side, and then separated and recovered from a concentrated radioactive liquid waste, and also nitric acid is formed at the anode side, and then separated and recovered from a concentrated radioactive liquid waste.    Non-Patent Document 1: S. Balagopal, et al., Selective sodium removal from aqueous waste streams with NaSICON ceramics, Separation and Purification Technology 15 (1999) 231-237.    Non-Patent Document 2: D. E. Kurath, et al., Caustic recycle from high-salt nuclear wastes using a ceramic-membrane salt-splitting process, Separation and Purification Technology 11 (1997) 185-198.    Non-Patent Document 3: D. T. Hobbs, Radioactive Demonstration of Caustic Recovery from Low-Level Alkaline Nuclear Waste by an Electrochemical Separation Process, WSRC-TR-97-00363 (1998).    Non-Patent Document 4: Proceeding of Efficient Separations and Processing Crosscutting Program, 1995, 1996, 1997.    Non-Patent Document 5: Ceramatec, Inc., Energy Efficient Process for Recycling Sodium Sulfate Utilizing Ceramic Solid Electrolyte, DOE contact No. DE-FCO2-95CE41158 (1999).    Patent Document 1: Japanese Unexamined Patent Application, First Publication No. 2000-321395    Patent Document 2: Japanese Unexamined Patent Application, First Publication No. Hei 4-283700